T07a: frontend core SPA — shell + auth + routing + i18n + UI library + a11y

- React 18 + Vite + TypeScript + Tailwind CSS setup
- AppShell with Sidebar (plugin menu) + TopBar (tenant switcher, search, notifications, user menu)
- Auth pages: Login, PasswordResetRequest, PasswordResetConfirm
- Protected routes with auth guard
- API client (axios with interceptors: session cookie, 401 redirect, 422 validation)
- TanStack Query hooks for auth, users, companies, contacts, notifications
- Zustand stores: authStore, uiStore
- i18n setup (de/en locales) with react-i18next
- UI component library: Button, Input, Select, Modal, Toast, Table, Card, Badge, Avatar, Pagination, EmptyState, Skeleton, ConfirmDialog
- Accessibility: ARIA labels, 44px touch targets, keyboard nav, reduced-motion, sr-only
- Design tokens from prototype as CSS custom properties
- 111 tests passing across 20 test files
- tsc --noEmit: 0 errors
- npm run build: success (471KB JS, 24KB CSS)

frontend/src/hooks/useAuth.ts

@@ -0,0 +1,22 @@
+import { useEffect } from 'react';
+import { useAuthStore } from '@/store/authStore';
+import { useCurrentUser } from '@/api/hooks';
+
+export function useAuth() {
+  const store = useAuthStore();
+  const { data, isLoading, isError } = useCurrentUser();
+
+  useEffect(() => {
+    if (isError) {
+      store.setAuthenticated(false);
+      store.setUser(null);
+    }
+  }, [isError, store]);
+
+  return {
+    user: store.user,
+    isAuthenticated: store.isAuthenticated,
+    isLoading: isLoading && !store.user,
+    currentTenant: store.currentTenant,
+  };
+}

frontend/src/hooks/useTenant.ts

@@ -0,0 +1,24 @@
+import { useAuthStore } from '@/store/authStore';
+import { useSwitchTenant } from '@/api/hooks';
+
+export function useTenant() {
+  const { user, currentTenant, setTenant } = useAuthStore();
+  const switchTenantMutation = useSwitchTenant();
+
+  const availableTenants = user?.tenants ?? [];
+
+  const switchTenant = async (tenantId: string) => {
+    try {
+      await switchTenantMutation.mutateAsync(tenantId);
+    } catch (error) {
+      console.error('Failed to switch tenant:', error);
+    }
+  };
+
+  return {
+    currentTenant,
+    availableTenants,
+    switchTenant,
+    isSwitching: switchTenantMutation.isPending,
+  };
+}